1. Field of the Invention
The present invention relates to a semiconductor module and a semiconductor device. More particularly, the present invention relates to a structure for separating dissipation of heat generated in a semiconductor device from grounding of a circuit.
2. Description of the Background Art
In semiconductor devices, in order to stabilize operation, it is required to efficiently release heat generated by semiconductor elements.
For example, Japanese Patent Laying-Open No. 4-174547 discloses a structure of a lead frame for a power semiconductor device. According to Japanese Patent Laying-Open No. 4-174547, a die pad of a lead frame is formed thicker than an outer lead of the lead frame. Heat generated in a semiconductor chip is absorbed in the die pad. The increased thickness of the die pad facilitates heat dissipation from the semiconductor chip. The surface of the die pad that is opposite to the surface provided with the semiconductor chip is exposed without being covered with resin. The exposed surface is brought into contact, for example, with a heat radiation fin, thereby facilitating dissipation of heat generated by the semiconductor chip.
Japanese Patent Laying-Open No. 6-61396 discloses a lead frame for improving a heat dissipation effect while the characteristics of a semiconductor device are maintained. This lead frame includes a stage on which a semiconductor chip is mounted. A heat sink plate is attached on the back surface of the stage.
Japanese Patent Laying-Open No. 2007-165442 discloses a mold package capable of improving heat dissipation and high-frequency characteristics. This mold package has a thick lead electrode connected with a semiconductor chip, and a thin lead electrode thinner than the thick lead electrode. The lower surface of the thick lead electrode is exposed on the lower surface of the package and is used as a heat dissipation electrode. On the other hand, part of the upper surface of the thick lead electrode is exposed on the upper surface of the package and is used as a grounding electrode.
For semiconductor devices operating at high frequency or high power, in particular, grounding of a semiconductor chip is important. In many cases, the back surface electrode of the semiconductor chip is used as a grounding electrode and is electrically connected with a lead frame via a die bonding pad. The potential of the grounding electrode can be stabilized more as the number of paths of ground current output from the back surface of the semiconductor chip is increased. However, Japanese Patent Laying-Open Nos. 4-174547 and 6-61396 do not disclose a specific structure for securing a path of ground current.
On the other hand, size reduction is required in a module including a circuit board and a semiconductor device operating at high frequency or higher power. According to the structure disclosed in Japanese Patent Laying-Open No. 2007-165442, the grounding electrode and the heat dissipation electrode are arranged on the upper surface and the lower surface of the package, respectively, so that ground current and thermal flow can be separated from each other such that the direction of ground current and the direction of thermal flow are opposite to each other. Thus, the module can be reduced in size. However, according to the structure disclosed in Japanese Patent Laying-Open No. 2007-165442, the path of ground current is limited to the path formed by the thick lead electrode. In view of stable operation of semiconductor devices, a larger number of ground paths is preferable.